In implementing enhanced programming in subscriber television systems, the home communication terminal (“HCT”), otherwise known as the set-top box, has become an important computing device for accessing media content services (and media content within those services) and navigating a user through a maze of available services. In addition to supporting traditional analog broadcast video functionality, digital HCTs (or “DHCTs”) now also support an increasing number of two-way digital services such as video-on-demand and personal video recording (PVR).
Typically, a DHCT is connected to a cable or satellite, or generally, a subscriber television system, and includes hardware and software necessary to provide the functionality of the digital television system at the user's site. Some of the software executed by a DHCT may be downloaded and/or updated via the subscriber television system. Each DHCT also typically includes a processor, communication components, and memory, and is connected to a television or other display device, such as a personal computer. While many conventional DHCTs are stand-alone devices that are externally connected to a television, a DHCT and/or its functionality may be integrated into a television or personal computer or even an audio device such as a programmable radio, as will be appreciated by those of ordinary skill in the art.
One of the features of the DHCT includes reception of a digital video signal as a compressed video signal. Another feature of the DHCT includes providing PVR functionality through the use of a storage device coupled to the DHCT. When providing this PVR functionality for analog transmission signals, compression is often employed after digitization to reduce the quantity or rate of data, and thus conserve data storage requirements for media content stored in the storage device. Video compression accomplishes this data reduction by exploiting data redundancy in a video sequence (i.e., a sequence of digitized pictures). There are two types of redundancies exploited in a video sequence, namely, spatial and temporal, as is the case in existing video coding standards. A description of some of these standards can be found in the following publications, which are hereby incorporated herein by reference: (1) ISO/IEC International Standard IS 11172-2, “Information technology—Coding of moving pictures and associated audio for digital storage media at up to about 1.5 Mbits/s—Part 2: video,” 1993; (2) ITU-T Recommendation H-262 (1996): “Generic coding of moving pictures and associated audio information: Video,” (ISO/IEC 13818-2); (3) ITU-T Recommendation H.261 (1993): “Video codec for audiovisual services at px64 kbits/s”; and (4) Draft ITU-T Recommendation H.263 (1995): “Video codec for low bitrate communications.”
One aspect of video signal digitization reduces the data rate of a video signal by performing downconversion of the color information of the video signal. The human eye has less spatial acuity to the color information than to the luminance (brightness) information. A video picture is transmitted with a brightness component (luminance) and two color components (chrominance). The digital representation of a video signal includes a luma signal (Y), representative of brightness, and color difference (or chroma) signals Cb (blue—Y) and Cr (red—Y). Without loss of generality to the specification of a video signal, luminance and luma are used interchangeably in this description as well as chrominance and chroma. The luminance information is often subjected to a non-linear transfer function, such as by a camera, and this process is called gamma correction.
Color formats for these three digitized channels of information, or equivalently the digitized YCbCr video signals, can include several forms to effect a spatial downsampling (or subsampling) of the chroma. These color formats include 4:4:4, 4:2:2, and 4:2:0, as described in ITU-601, which is an international standard for component digital television that was derived from the SMPTE (Society of Motion Picture and Television Engineers) RP1 25 and EBU 3246E standards and which are herein incorporated by reference. ITU-/601 defines the sampling systems, matrix values, and filter characteristics for Y, Cb, Cr and red-green-blue (RGB) component digital television. ITU-601 establishes a 4:2:2 sampling scheme at 13.5 MHz for the Y signal and 6.75 MHz for the CbCr signals with eight-bit digitizing for each channel. Certain implementations may process each channel internally with a higher precision than eight bits. These sample frequencies were chosen because they work for both 525-line 60 Hz and 625-line 50 Hz component video systems. The term 4:2:2 generally refers to the ratio of the number of Y signal samples to the number of CbCr signal samples in the scheme established by ITU-601. For every four Y samples, the CbCr signals are each sampled twice. On a pixel basis, this can be restated as for every pixel pair, there is a sample Y1, Y2, and a CbCr shared among the two luma samples. Equal sampling (i.e., Y1CbCr, Y2CbCr) at, say 4:4:4, is simply not required for most subscriber television systems due to the reduced visual acuity for color information. Consequently, the 4:2:2 format is an industry standard for input from a digitizer such as an analog video signal decoder and for output to an analog video signal encoder that drives a television display.
MPEG-2 (Motion Pictures Expert Group) main profile uses 4:2:0 color formatting, which reduces transmission bandwidth and data storage requirements, since now for every four luma samples, the CbCr signals are each sampled once. According to ITU-601, the first number of the 4:2:0 color format (i.e., “4”) historically represents approximately “4” times the sampling rate for a video signal. The second number (i.e., “2”) represents a defined horizontal subsampling with respect to the luma samples. The third number (e.g., “0”) represents a defined vertical subsampling (e.g., 2:1 vertical subsampling).
Thus, in the process of digitizing a video signal, the DHCT downconverts the CbCr signal components (or chroma) from the 4:2:2 color format to the 4:2:0 color format using filtering technology well-known in the art to comply to the format required for compression, which also reduces data storage consumption in the storage device. Likewise, a remote compression engine (i.e., remote from the DHCT) may likely have to perform downconversion from the 4:2:2 color format to the 4:2:0 color format using filtering technology. In transitioning from a 4:2:2 color format to a 4:2:0 color format, the chroma signal has to represent a larger pixel sampling area.
Displaying the video pictures on a display device includes the process of receiving a transmitted digital video signal or the process of retrieving the pictures of the video signal from the storage device, decompressing the resultant video signal and reconstructing the pictures in memory, and upconverting the CbCr signal components (or chroma) from the 4:2:0 color format to the 4:2:2 or 4:4:4 color format for eventual display to a display device, or for other processing. Thus, conversion again is needed to reconstruct the color information of the original signal, a process that often reduces picture quality. What is needed is a system that reduces the loss of picture quality (or preserves the picture quality) in the conversion of the chroma.